Presence of intact endothelium is the best prevention of narrowing or occlusion of the vascular lumen by thrombosis, or by atherosclerotic intimal thickening, or by spasms. To avoid the occurrence of these common pathogeneic mechanisms of the most prevelent cardiovascular diseases (ischemic disease of heart, brain, periphery), endothelium has to be not only morphologically intact (physically present, without "injury" in the sence of denudation), it also must function properly. In the past grant periods we have learned to analyze quantitatively the interdependent cell movements which restore the morphologic integrity of the endothelium. We now propose to continue these studies, concentrating on the intercellular communications which coordinate individual cell movements into highly organized, efficient maintenance and repair mechanisms. We have also recognized movement of living cells as an extremely sensitive indicator of vital cell functions in the undisturbed endothelium. Thus we have a new tool to explore quantitatively subtle functional alterations of endothelium, and we can test a new hypothesis which claims that certain cellular movements which we have observed at the border between individual cells may be involved in the control of endothelial permeability to molecules as well as to cells.